The present invention relates to pneumatic radial-ply runflat tires and in particular to runflat tire construction that reduces lateral and circumferential tread lift during runflat operation.
Designers of pneumatic tires have devised various methods by which to make a tire that is capable of providing acceptable performance following sudden, unexpected deflation, such as a tire puncture. Under such conditions, the goal of the tire designer is to design a tire able to provide adequate vehicle handling and safe continued operation over a distance from the place where the tire has lost its pressure to a place desired by the driver, such as a service station where the tire can be repaired or replaced.
A pneumatic tire designed for sustained operation under conditions of unpressurization or underpressurization is generally called an extended mobility tire (EMT) or simply a runflat tire. The latter term refers to the tire""s ability to be driven while uninflated or in what would generally be called a xe2x80x9cflatxe2x80x9d condition. A conventional pneumatic tire will collapse upon itself, becoming flat, when it is uninflated and carrying the weight of a vehicle. Such a tire""s sidewalls buckle axially outward in the portion of the tire adjacent to the ground-contacting portion of the tread, making the tire xe2x80x9cflat.xe2x80x9d
A tire""s sidewalls are the portion of a tire usually having the lowest resistance to deformation under vertical loading. Therefore the most widely used design methods used to achieve acceptable extended mobility or runflat capability involve various methods by which to reinforce the sidewalls, giving them sufficient strength and rigidity to support the wheel load when the tire is uninflated. Such reinforced sidewalls do not collapse or buckle onto themselves.
State-of-the-art reinforced sidewalls contain one or more circumferentially disposed wedge inserts within each sidewall region, such as between the carcass plies. Thus each sidewall is thickened in such a way that its overall thickness in the region between the bead and the tread shoulder is more or less uniform. The one or more circumferentially disposed wedge inserts in each sidewall are generally crescent-shaped in cross-sectional view, in order to conform to the internal shape of the sidewalls. Such wedge reinforced sidewalls, when operated in the uninflated condition, experience a net compressive load in the region of the sidewall. And, more specifically, the bending stresses on the sidewalls are such that the axially outwardmost portions of the reinforced sidewalls experience tensile forces while the axially inward portions experience compression stresses during runflat operation.
The bending forces that act upon the reinforced sidewalls of an uninflated EMT or runflat tire lead to the transmission of bending forces from the sidewall regions to the ground-contacting portions of the tread.
The result is that the uninflated EMT tends to have a compressed tread footprint. More specifically, the transmission of bending forces from the sidewalls to the tread region tend to cause the central portions of the tread to buckle upwards. The term or art used to refer to the upwards buckling of the tread is tread lift.
The tread lift arises from compressive loading in both the lateral and circumferential directions. The central portion of the tread accordingly tends to lose contact with the ground during runflat operation, having consequent adverse effects upon vehicle handling, especially during high speed operation.
Tread lift also takes place in the circumferential direction. The forces acting upon the ground-contacting portion of the tread are such that the bending forces that act upon the tread in the frontmost and rearmost portions of the footprint area act in such a way that, as viewed from the side of the tire, i.e., in the axial direction, the central region of the ground-contacting portion of the tread tends to rise off the ground. Thus tread lift has both lateral and circumferential components.
One method used to stiffen the tread against lateral tread lift employs the use of a metal-reinforced first radial carcass ply which resists the tensile forces that arise during lateral tread lift. Another method by which to inhibit tread lift involves the use of decoupling grooves in the lateral-most regions of the tread. The decoupling grooves inhibit the transmission of bending forces from the sidewalls to the central portions of the tread. The bottoms or radially inwardmost portions of the decoupling grooves act as hinges, allowing the sidewalls to deform without affecting the adjacent tread region. An example of a runflat tire design incorporating decoupling xe2x80x9cshoulder groovesxe2x80x9d is described in Goodyear""s Patent Application PCT/US98/00717, filed Jan. 15, 1998 and having a common assignee with the present application.
Another method by which to inhibit runflat tread lift is described in Goodyear""s Patent Application PCT/US98/14452, filed Jul. 10, 1998 and having a common assignee with the present application, wherein a lateral-tensile-stress bearing fabric underlay is deployed radially inward of the breaker package. The laterally aligned (or axially aligned) fibers of such a fabric underlay might or might not be prestressed in tension to inhibit upward buckling of the central portion of the tread of an uninflated EMT or runflat tire. Yet another method by which to minimize runflat tread lift is described in Goodyear""s Patent Application PCT/US98/06004, filed Mar. 26, 1998 and having a common assignee with the present application, wherein a xe2x80x9ctread wedgexe2x80x9d is used to thicken the structures underlying the tread and thus inhibit tread lift during runflat operation.
With regard to the circumferential component of tread lift during runflat operation, one method of managing it is described in the aforementioned Goodyear""s Patent Application PCT/US98/06004 wherein a fabric underlay comprising more or less circumferentially aligned cords inhibits tread lift in the circumferential direction.
As always in the design of an EMT or runflat tire, the goals of the tire design include the minimizing of tire mass and provision of good riding comfort under normal inflated operation while also providing adequate safe vehicle handling during uninflated operation. Long runflat service life is also a design goal.
It is an object of the present invention to provide a radial tire as defined in one or more of the appended claims and, as such, having the capability of being constructed to accomplish one or more of the following subsidiary objects.
One object of the present invention is to provide a runflat radial tire that is resistant to both lateral and circumferential tread lift during runflat operation.
Another object of the present invention is to provide a runflat radial tire that is relatively light in weight.
Another object of the present invention is to provide a runflat tire that is able to provide good vehicle runflat handling and an extended runflat service life.
Still another object of the present invention is to provide a runflat tire having a reduced heat generating potential within the tread reinforcing structures during both normal inflated, high-speed operation and especially during runflat operation.
Yet another object of the present invention is to provide a plurality of tread stiffening support that provide puncture protection from cuts and objects picked up on the road.
The present invention relates to a pneumatic radial ply tire having a tread, a carcass comprising a radial ply structure, an inner liner and two sidewalls each reinforced by one or more wedge inserts, a belt structure located between the tread and the radial ply structure and a plurality of parallel-aligned ribs lying laterally across and radially inward of the tread and radially inward or outward of the belt structure. The length of each rib of the plurality of ribs is approximately equal to the width of the belt structure. In one embodiment, each rib is made of a monolithic, fiber-reinforced plastic material from the class of plastic matrix materials that includes thermoplastics, such as polyetherirnide, polyetheretherketone and polyphenylene sulfide and thermosets, such as polyester and epoxy. The fiber-reinforcing materials are of the class of high-modulus materials that includes carbon fiber, fiberglass and aramid filaments. Preferably, the formulation of the fiber-reinforced plastic material is between about 30 percent to 70 percent by weight of reinforcement material and the remainder primarily being the plastic matrix. The reinforcing fibers within each monolithic rib are preferably unidirectional, being aligned with the axis, extending along the length of the rib. The preferred relationship between the thickness and width of each rib is such that the thickness is between 10 percent and 45 percent of the width of the rib. The minimum preferred circumferential spacing between each rib in the plurality of ribs is between 50 percent and 200 percent of the width of the ribs on either side of the spacing. An alternative location of the plurality of ribs is beneath or radially inward of the tread and belt structure and within the ply structure.
An alternative embodiment incorporates a rib made of three layers of which the first and third layers, i.e., the two outermost layers, are made of a reinforced plastic material as previously described with regard to the first embodiment. The middle layer of the three rib layered rib is made from an elastomeric material, such as urethane or rubber, to allow the ribs to endure shear stresses associated with the tread deflections during normal or runflat operation of the tire.
Other benefits and advantages of the invention will become apparent to those skilled in the art to which it pertains upon a reading and understanding of the following detailed specification.